Neutralizing system



Sept. 5, 1933. w. VAN B. ROBERTS 1,925,568

NEUTRALIZ'ING SYSTEM Filed Jan. 7, 1927 T 1 1 I I W 1 1 INVENTOR WALTERVAN B. ROBERT TORNEY Patented Sept. 5, 1933 UNITED STATES PATENT OFFICEQ 1,925,568 a I I I NEUTRALIZING SYSTEM Walter van B. Roberts,Bronxville, N. Y., as-

signor to Radio Corporation of America, a cor-- poration of Delaware i IApplication January 7, 1927. Serial No 159,532 '3 Claims. (01. 179171)This invention relates to amplifiers, detectors, repeaters, or otherequivalent arrangements. In general it relates to arrangements whereinenergy is introduced, at one set ofterminals and 5 taken off at anotherset of terminals in a changed condition. The change may be either ofcurrent voltageor power characteristics. The particular device relatesto amplifier circuits wherein an inherent capacity coupling exists whichproduces undesirable results and means are provided forcounterbalancingor neutralizing this capacitycoupling. s

It will perhaps be appreciated in the present state of the art of vacuumtube circuits that whenever an undesirable feed-back coupling or such acoupling as to'progressively increase its effect of any oscillationslonthe input circuit from the output circuit of an amplifying device insuch a way as to produce spontaneous oscillations; that a magneticcoupling between the input circuit and the output circuit may be usedwhich will react on the input circuit so as to' cut down feed-back orreactive effect to a degree .below the limits at which spontaneousoscillations are effective. Various arrangements have heretofore beeneffective and particularly disclosed and described in various patents.

The patent to Hartley 1,183,875 dated May 23, 1916, shows one type ofarrangement wherein a counter-feed-back is usedto reduce the amount ofdirect feed-back. In the same way: this invention uses acounter-feed-back. coupling. The present invention is novel in thefactthat it is rigorously a Wheatstones bridge. On account of thispossibility of perfect balance, it is possible to operate a regenerativedetector after a blockingstage of amplification and use. a very criticalamount of regeneration without stirring up current'in the amplifierinput and thus throwin the detector in and out of oscillation.

In analyzing the undesirable oscillations used in an amplifieror-equivalent arrangements, two general features must be taken intoconsiderationr First, the arrangement by itselfoftentimes has a tendencyto produce spontaneous self-sustained oscillations'which will continueof their own accord if suflicient coupling is present. Secondly,undesirable oscillations may be introduced into the output connectionsof the device which will react through the arrangement and into anyother means which may be connected to the input connections. With thepresent arrangement this second difiiculty has been over-' come so thatboth spontaneous.self-sustained oscillations and reactiveoscillations'on the output introduced between the filament and plate.'The circuit are prevented from effecting the input circuit to. such adegree as to effect any means connected tothe input circuit.

It is understood how a'capacity coupling oftentimesexists between thegrid and plate of a three 6 or more element vacuum tube which producesundesirable effects betweenthe input and output circuits. In thisinvention this capacity coupling is counter-balanced or neutralized by aproportional capacity coupling between the plate and 5 cathode of thevacuum device. This capacity coupling bears a relation to the naturalcapacity coupling between the plate and grid determined by the magneticcouplings between the two circuits. 1

Further and more definite objects will be apparent in connection withthe'following specification,.claimsand drawing of which Fig. 1represents a simple adaptation of my arrangement,

Fig. 2 shows the equivalent bridge for the purpose of analyzing thetheoretical'balance of the system, and

Fig. 3 shows one way a series of such amplifiers I may be coupledtogether to produce several stages of amplification.

Fig. 4 shows a variation in construction.

Fig. 5 shows still another arrangement 'Fig. 6 showshow a plurality oftubes may be used with a common battery.

Referring more particularly to Fig. 1, l represents the tuningarrangement shown as acondenser but might as well beany other equivalentarrangement, 2 and 3 make up the corresponding element which is shown asan inductive coil with an associated adjustable tap 4 directly connectedto the battery 5 and the output coil 6. The grid and lfilament of thevacuum tube 7 as indicated as F and G are directly connected to theopposite terminals of the coils 2 and 3.

Assuming that the capacity 8 represented by dotted lines is a naturalinherent capacity between the grid and plate elements of the vacuum tube7 then a corresponding capacity 9 may be size of this second capacity 9will be determined by the degree of coupling between the grid and platecircuits through the coupling coils 2 and 3. It can be readily seen thatif the tap 4 is adjusted to the midpoint between the grid and thefilament and the external capacity 9 is the same as the natural capacity8, then any current which happens to be flowing in the output'coil 6 dueto voltage applied to coil 6 will divide equally at the tap 4; partflowingthrough the capacity 8 and part flowing through the capacity 9.Of course, this will not be true if current is flowing between thefilament and any other electrode so as to upset the balance but thiswill be true for all oscillations that are produced externally as fromcircuit 10 into the coil 6 regardless of the oscillations produced inthe coil 6 through the operation and functioning in the proper manner ofthe vacuum tube 7.

In this way it can be seen that extraneous, undesirable oscillations areprevented from affecting any circuit which happens to be coupled to theinput circuit of the tube 7. In addition to this it can also be seenthat the coupling between section 3 of the coil 2 which includes theoutput circuit will produce a counterfeed-back into the.

grid circuit so that spontaneous sustained oscillations of the tubeitself will be prevented. In this way both types of oscillations may beeliminated from the circuit.

If we substitute equivalent elements of the circuit shown in Fig. 1 intoan equivalent bridge arrangement it will result in a diagram somewhatsimilar to that shown in Fig. 2 where F, G and P indicate filament, gridand plate, respectively and the tap is indicated at 4, separating thepart 3 from'the coil 2. The inherent capacity between grid and plate isindicated at 8 and the introduced counter-balancing capacity at 9. Nowif oscillations are introduced on the coil 2, 3 through the operation ofthe tube they will necessarily be reproduced in the coil 11 but onaccount of the counter-feed-back qualities of a coupling between section3 and the coil 2 no self-sustained oscillations of thetube will bepermitted and also any oscillations which might be introduced on thecoil 11 will be balanced out between the terminals G and F on account ofthe-inherent symmetry of the system.

This arrangement may also be adapted for use with a multistage amplifierarrangement such as is shown in Fig. 3. Where 1, 2, and 3 are,respectively, similar elements as are shown in Fig. 1,.

but instead of having a direct connection from the plate'to the taps 4,a large capacity condenser 12 is used. These condensers 12 prevent thefiow of .high potential directly from the plate connections to the taps4 from the filament circuits which would, in this way, cause a shortcircuit of the direct current flow. Also choke coils 13 may be used toprevent short circuit of the oscillating current produced in the vacuumtubes. In this way oscillations may be introduced in the coils 2, 3transferred through the coupling 14- to another set of coils 2, 3amplified again and taken off through the coupling 15. The highpotential battery may be connected to the terminals 16 and 1'7 asindicated by plus B and minus B.

Another way of expressing the method of neutralization might be asfollows:--If full signal strength is applied between the grid andfilament to coils 2 and 3 and the rotor of the tuning condenser is atfilament potential, eliminating hand capacity .effects'then at the sametime current in the output circuit will have no effect upon the apotential dilference between the grid and the rate means. The spacecurrent, that is the current which flows in the ordinary sense betweenthe filament and plate of the vacuum tube will, through direct magneticcoupling of the coil 2, counteract the effect of the capacitativecoupling between grid and plate while the current pro duced byextraneous application of voltage will be counteracted through the twocircuits leading to imaginary condenser 9 connected between the plateand filament thereby maintaining perfect balancing. If the magneticcoupling is so chosen as to prevent undesirable oscillations at thehighest frequency of the range used, then it will also preventoscillations at any other frequency and the neutralization for externalsources of potential will be automatically taken care of on account ofthe symmetry of the arrangement.

However, it is not intended to be limited to the used of a connectiontap 4 located exactly in the middle of the coil 2 but it might as wellbe at any convenient point along the coil, provided, the condenser 9were adjusted to compensate for the difference produced by theunbalance. It is even advantageous in practice to use a tapquite closeto the filament end of the coil or a separate feedback coil of a fewturns compared tothe number of turns of the grid coil. Other equivalentsmight as well be substituted for those shown to the extent which suchdevices are well known in the art. In Fig. 4 the B battery may beconnected direct to filament, so as to be available for more than onetube. Electrically this figure is the-same as Fig. 1 with the additionof coil 5. If the arrangement of Figure 5 is used, the input couplingcoil 10 may be coupled to any part of coil 2, as no current is producedin any part of coil 2 by voltage introduced into the load circuit- Fig.6 is another way of making this possible. Fig. 5 also shows the steadycurrent supplied to the platethrough a choke coil 13, whose apparentcapacityto high frequency currents is considered to be part of condenserl.

It is advisable to make capacity 4 much larger than capacity 3, as thisreduces the feed back of voltage generated by the tube, and also allowsthe use of a very'high resistance at'Z, in fact, makes the addition ofresistance 7 unnecessary in most practical work. r

In Fig. 6 the application of the invention is to to receiver employing aone way repeater followed by a regenerative or oscillating detector.More than one stage .of amplification may be used if desired. When thefilament is lighted, the interpedances between plate and griclandbetween plate and filament. A blocking condenser 8.. is inserted to keepthe constant potential of the plate 011 the grid. 1% is an auxiliaryby-pass condenser and 15 represents the primary to thedetector in serieswith counter feedback coil 5.

This invention enjoys the following advantages: great stability, therotor of the input tuning condenser is at filament, potential, anytypeof load or output circuit may be used, and it is comparatively free frompossibility of parasitic oscillations. e l

Having described my invention, I claim:

1. In a wave repeating system, a space dis charge device forrepeatingpotential variations comprising an input and an outputelectrode between which two. electrodes appreciable capacity exists anda-filamentary cathode, a pair of condensers in series connected betweenthe input electrode and said filamentary cathode, an output circuit forutilizing the variation repeated by said device, said output circuitbeing connected across the output electrode and the common point of saidtwo condensers, a counterbalancing coupling between the filamentarycathode and the output electrode and a source of current in series witha choke coil connected between the filamentary cathode and the outputelectrode.

2. In a wave repeating system a space discharge device for repeatingpotential variations having a filamentary cathode and an input and anoutput electrode between which appreciable capacity exists, a tunedinput circuit adapted to have potential variations impressedthereon, an

output circuit for utilizing said potential variations, a pair ofreactances in series connected between the input electrode and saidfilamentary cathode, said output circuit being connected between the.output electrode and a point intermediate said two reactances, acapacitive counterbalancing coupling between the filamentary cathode andthe output electrode and a source of current connected between thefilamentary cathode and the output electrode.

3. In a wave repeating system, a space discharge device for repeatingpotential variations having a filamentary cathode and an input and anoutput electrode between which appreciable capacity exists, a tunedinput circuit adapted to i have potential variations impressed thereon,an outputcircuit for utilizing said potential variations, a pair ofreactances in series connected between the input electrode and saidfilamentary cathode, said output circuit being connected between theoutput electrode and a point intermediate said tWo reactances, acapacitive counter-balancing coupling between the filamentary cathodeand the output electrode, a source of current connected between thefilamentary cathode and the output electrode and an impedance in serieswith a blocking condenser connected across said input and outputelectrodes.

WALTER VAN B. ROBERTS.

